1. Field of the Invention
The present invention relates to an image forming apparatus for creating an image using an image-carrying belt.
2. Related Art
Some electrophotographic image forming apparatuses such as copiers and printers use belt-like image carriers (hereinafter referred to as image-carrying belts) to create images.
In this kind of image forming apparatus, latent electrostatic images are created on a photoconductor and developed with toners, thus visualizing the toner images. The toner images are then transferred to an image-carrying belt. Subsequently, all the toner images on the image-carrying belt are transferred to paper (recording medium) at a time. This method is normally known as the "intermediate transfer method", and the image-carrying belt is termed the "intermediate transfer belt".
In a full-color copier or the like making use of the aforementioned transfer method, toner images of plural colors (e.g., yellow (Y), magenta (M), cyan (C), and black (K)) are superimposed in turn on the image-carrying belt and transferred, thus forming one color image. At this time, if the toner images of the various colors are not coincident in transfer position, or if the concentrations of the toner images deviate from specified concentrations, deviations from the correct colors and nonuniformities of colors take place. Accordingly, where a color image is created, an ineffective region separate from the normal toner image formation region from which toner images are transferred to the paper is formed on the image-carrying belt. Patch images (referred to as the process control patch images) for position registration of toner images and for control of the concentrations are formed. Various control parameters are appropriately set or modified according to the results of reading of the patch images. In this way, the color image is free of color shifts and color nonuniformities.
A primary transfer position where toner images are transferred from the photoconductor to the image-carrying belt and a secondary transfer position where toner images are transferred from the image-carrying belt to the paper are established in the path in which the image-carrying belt moves. Transfer rolls or transfer means such as transfer belts are mounted in these two transfer positions. The transfer roll placed in the primary transfer position is pressed against the surface of the photoconductor via an image-carrying belt. The transfer roll placed in the secondary transfer position is held so as to be capable of being brought into and out of contact with a backup roll that supports the image-carrying belt from inside.
In the secondary transfer position described above, the transfer roll is pressed against the front side (to which toner images are transferred) of the image-carrying belt held by the backup roll. Under this condition, a transfer bias voltage is applied to one or both of the backup roll and the transfer roll. Thus, the toner images are transferred from the image-carrying belt to the front surface of the paper.
The process control patches formed in the ineffective regions of the image-carrying belt directly touch the transfer belt without via the paper and so the toners forming the process control patches adhere to the transfer roll from the image-carrying belt. On the other hand, when the paper is nipped between the image-carrying belt and the transfer roll, the transfer roll is pressed against the rear surface of the paper. Therefore, the toner adhering to the transfer roll as described above is again transferred to the rear surface of the paper. This contaminates the rear surface of the paper. Especially, in the case of process control patches, their sharp edges are accentuated. If they are transferred to the rear surface of the paper as they are, the contamination becomes worse.
Besides the process control patches, if the toner images carried on the image-carrying belt differ in size from the paper to which the toner images should be transferred, the toners on the belt adhere to the transfer roll. This may contaminate the rear surface of the paper. Furthermore, toners adhering to a component other than the image-carrying belt such as an image carrier that carries images on the photoconductor may adhere to the transfer roll, thus contaminating the rear surface of the paper.
A countermeasure against these problems is disclosed in Japanese Unexamined Patent Publication No. 328401/1996. In particular, a bias voltage that is opposite in polarity to the transfer bias voltage applied in transferring toner images from an image-carrying belt to paper (hereinafter referred to as a reverse bias) is applied to the transfer roll, thus cleansing the toner adhering to the transfer roll.
Japanese Unexamined Patent Publication No. 218623/1997 discloses a technique for removing toners from the surface of a transfer roll by pushing a cleaning blade against the transfer roll.
The techniques disclosed in these two Japanese Patent Publications have the following problems. First, in the technique disclosed in Japanese Unexamined Patent Publication No. 328401/1996, the cleaning capability is affected by paper jams and environmental variations such as temperature and humidity variations. Consequently, it has been difficult to completely cleanse the toners simply by applying a reverse bias to the transfer roll.
The technique disclosed in the above-cited Japanese Unexamined Patent Publication No. 218623/1997 has the following problems. (1) Since the blade is pushed against the transfer roll, its surface is deteriorated, thus shortening the life of the transfer roll. (2) The surface of the transfer roll is coated with a fluorocarbon or the like to improve the ability to peel off from the paper and the cleanability. This coating is gradually peeled by the blade. Therefore, the paper is wound around the transfer roll. Hence, jams tend to occur. (3) It is necessary to install a new large-sized recovery box for recovering toner removed by the blade. (4) The surface of the transfer roll including the coating wears down. Also, the blade itself wears down. In consequence, the cleaning effect of the blade deteriorates.